1. Field of the Invention
The invention concerns a process for the manufacture of flexible polyurethane foams with improved resistance to hydrolysis and good die cutability wherein liquid polyester polyols with hydroxyl numbers of 40 to 80 and molecular weights of 1500 to 5000 are used as the polyhydroxyl compounds. The polyhydroxyl compounds are prepared by the polycondensation of organic dicarboxylic acids with either polyol mixture A which contains 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, at least one triol and optionally diethylene glycol or polyol mixture B which contains 1,4-butanediol, 1,6-hexanediol, diethylene glycol and at least one triol.
2. Prior Art
The preparation of flexible polyurethane foams is basically well known to those skilled in the art and is described in detail in the appropriate literature, for example, the Plastics Handbook, Volume VII, "Polyurethane" by R. Vieweg and A. Hoechtlen, Carl Hanser Publishers, Munich, 1966.
In addition to catalysts, blowing agents, as well as auxiliaries and additives, toluene diisocyanates are commonly used as the polyisocyanates in the preparation of polyurethane flexible foams. Mixtures of toluene diisocyanates, diphenylmethane-diisocyanates and polyphenyl-polymethylene-polyisocyanates are used in order to reduce the toxicity.
Preferably used as the polyhydroxyl compounds are polyether polyols, particularly those based on ethylene and/or propylene oxide. Drawbacks of the polyurethane flexible foams produced from these materials are that the physical properties do not meet the requirements of all areas of application and the foams are susceptible to degradation due to light and various oxidation agents.
If special physical properties of the polyurethane flexible foams are required instead of polyether polyols, polyester polyols may be used in their preparation. The resultant foams, however, have a very low resistance to hydrolysis.
In order to reduce these drawbacks, it is suggested in German Published Application No. 28 15 540 that certain starting components be selected for the preparation of polyurethane flexible foams. These include a urethane group-containing mixture of diphenylmethane diisocyanates and polyphenyene-polymethylene-polyisocyanates containing 55 to 85 percent by weight of diphenylmethane-diisocyanate isomers as the polyisocyanates, polyester polyols or mixtures of polyester polyols and polyether polyols containing at least 60 percent by weight of polyester polyol as the polyhydroxyl compounds, and as the blowing agent, water or mixtures of water and low boiling, halogen-containing hydrocarbons. Particular mention is made of polyester polyols having a molecular weight of 750 to 5000 and a functionality of 2 to 3.5 which are produced by the polycondensation of a dicarboxylic acid mixture of succinic, glutaric and adipic acid in certain quantity ratios and 2- and 3-functionality alcohols. In this manner polyurethane flexible foams can be produced which have high load bearing and energy absorption capacities as well as resistance to hydrolysis.
Polyurethane flexible foam parts having unusual shapes may best be produced by cutting the parts from foils and panels using specially developed equipment, i.e., die cutters.
It is known that it is very difficult to cut low density polyurethane flexible foams based on polyester polyol. The cutting surfaces adhere to each other at certain points or totally so that a flawless structural manufacture of complicated parts is almost impossible. The addition of small quantities of paraffin oil in order to improve the cutability is impractical since paraffin oil and polyester polyols are not miscible and therefore flaws result in the formation of the foam.
It is therefore the purpose of this invention to produce polyurethane flexible foams from starting components which were liquid at the processing temperature and which have improved physical properties, good cutting properties and high resistance to hydrolysis.
Surprisingly, this task could be met by using specific polyester polyols based on dicarboxylic acids and mixtures of diols and triols in certain quantity ratios for the preparation of the polyurethane flexible foams.